The invention generally relates to preconditioning spins near a nuclear magnetic resonance (NMR) region.
Nuclear magnetic resonance (NMR) may be used to determine properties of a sample, such as body tissue (for medical imaging purposes) or a subterranean formation (for well logging purposes). For example, for the subterranean formation, NMR may be used to determine and map the porosity, formation type, permeability and oil content of the formation.
Referring to FIG. 1, as an example, NMR may be used in a logging-while-drilling (LWD) operation to map the properties of a subterranean formation 10. In this manner, an axisymmetric NMR tool 6 may be part of a drill string 5 that is used to form a borehole 3 in the formation 10. The tool 6 may be, as examples, one of the tools described in Sezginer et. al., U.S. Pat. No. 5,705,927, entitled “Pulsed Nuclear Magnetism Tool For Formation Evaluation While Drilling Including a Shortened or Truncated CPMG Sequence,” granted Jan. 6, 1998; Miller, U.S. Pat. No. 5,280,243, entitled “System For Logging a Well During the Drilling Thereof,” granted Jan. 18, 1994.
The NMR measuring process is separated by two distinct features from most other downhole formation measurements. First, the NMR signal from the formation comes from a small resonance volume, such as generally thin resonance volume 20a (see FIG. 2), and the resonance volume 20a may have a radial thickness that is proportional to the magnitude of a {right arrow over (B)}1 magnetic field (not shown). Depending on the shape of the resonance zones, the volume may extend, as an example, from as little as 1 millimeter (mm) in one direction and as long as several inches in another. Secondly, the NMR measurement may not be instantaneous. Both of these facts combined make the NMR measurements prone to tool motions, such as the NMR tool 6 moving around the periphery of the borehole 3, as further described below.
To perform the NMR measurements, the NMR tool 6 may include permanent magnets to establish a static magnetic field called {right arrow over (B)}0 (not shown); a radio frequency (RF) coil, or antenna, to radiate the time varying magnetic field {right arrow over (B)}1 that is perpendicular to the {right arrow over (B)}0 field; and an RF coil, or antenna, to receive spin-echoes from the formation in response to an NMR measurement, as described below. These two coils may be combined into a single transmit/receive antenna.
As an example, the NMR tool 6 may measure T2 spin-spin relaxation times of hydrogen nuclei of the formation 10 by radiating NMR detection sequences to cause the nuclei to produce spin-echoes. The spin-echoes, in turn, may be analyzed to produce a distribution of T2 times, and the properties of the formation may be obtained from this distribution. For example, one such NMR detection sequence is a Carr-Purcell-Meiboom-Gill (CPMG) sequence 15 that is depicted in FIG. 4. By applying the sequence 15, a distribution of T2 times may be obtained, and this distribution may be used to determine and map the properties of the formation 10.
A technique that uses CPMG sequences 15 to measure the T2 times may include the following steps. In the first step, the NMR tool 6 transmits the {right arrow over (B)}1 field for an appropriate time interval to apply a 90° excitation pulse 14a to rotate the spins of hydrogen nuclei (that are initially aligned along the direction of the {right arrow over (B)}0 field) by 90°. Although not shown, each pulse is effectively an envelope, or burst, of an RF carrier signal. After the spins are rotated 90° from the direction of the {right arrow over (B)}0 field, the spins immediately begin to precess in the plane perpendicular to the {right arrow over (B)}0 field at first in unison, then gradually losing synchronization. For step two, at a fixed time T following the NMR pulse 14a, the NMR tool 6 pulses the {right arrow over (B)}1 field for a longer period of time (than the NMR pulse 14a) to apply an NMR refocusing pulse 14b to rotate the precessing spins through an additional angle of 180° with its carrier phase shifted by ±90°. The NMR pulse 14b causes the spins to resynchronize and radiate an associated spin-echo 16 (see FIG. 5) which peaks at a time approximately equal to T, after the 180° refocusing NMR pulse 14b. Step two may be repeated “k” times (where “k” is called the number of echoes and may assume a value anywhere from several hundred to as many as several thousand, as an example) at the interval of te (approximately 2·T). For step three, after completing the spin-echo sequence, a waiting period (usually called a wait time) is required to allow the spins to return to equilibrium along the {right arrow over (B)}0 field before starting the next CPMG sequence 15 to collect another set of spin-echoes. The decay of each set of spin-echoes is observed and used to derive the T2 distribution.
The T2* time characterizes a time for the spins to no longer precess in unison after the application of the 90° excitation pulse 14a. In this manner, at the end of the 90° excitation pulse 14a, all the spins are pointed in a common direction perpendicular to the static B0 field, and the spins precess at a resonance frequency called the Larmor frequency for a perfectly homogenous field. The Larmor frequency may be described by {right arrow over (ω)}0=γ{right arrow over (B)}0, where γ is the gyromagnetic ratio, a nuclear constant. However, the {right arrow over (B)}0 field typically is not homogenous, and after excitation, the spins de-phase with T2* due to inhomogenieties in the static {right arrow over (B)}0 field. This decay is reversible and is reversed by the refocusing pulses 14b that cause the echoes. In addition, irreversible de-phasing occurs (spin-spin relaxation) and is described by the T2 time constant. This results in the decay of successive echo amplitudes in the CPMG sequence according to the T2 time constant. With “inside-out” NMR, typically, spins are measured with T2 >>T2*.
As stated above, the distribution of the T2 times may be used to determine the properties of the formation. For example, referring to FIG. 6, the formation may include small pores that contain bound fluid and large pores that contain free, producible fluid. A T2 separation boundary time (called TCUT-OFF in FIG. 6) may be used to separate the T2 distribution into two parts: one part including times less than the TCUT-OFF time that indicate bound fluids and one part including times greater than the TCUT-OFF time that indicate free, producible fluids.
Each T2 time typically is computed by observing the decay of the spin-echoes 16 that are produced by a particular CPMG sequence 15. Unfortunately, the drill string 5 (see FIG. 1) may experience severe lateral motion. However, the T2 time is approximately proportional to another time constant called a T1 spin-lattice relaxation time. The T1 time characterizes the time for the spins to return to the equilibrium direction along the {right arrow over (B)}0 field, and thus, considering both the T1 and T2 times, each spin may be thought of as moving back toward the equilibrium position in a very tight pitch spiral during the T1 recovery. Fortunately, the T1 and T2 times are approximately proportional. As a result, the T2 distribution may be derived from measured T1 times. In fact, the original work on establishing bound fluid cutoffs was done using T1. Those results were then expressed and used commercially in terms of T2. See W. E. Kenyon, J. J. Howard, A. Sezginer, C. Straley, A. Matteson, K. Horkowitz, and R. Ehrlich, Pore-Size Distribution and NMR in Microporous Cherty Sandstones, Paper LL (paper presented at the 30th Annual Logging Symposium, SWPLA, Jun. 11-14, 1989).
Polarization-based measurements may use either inversion recovery sequences or saturation recovery sequences. With the saturation recovery sequences, the spin system is saturated, e.g. with several 90° pulses that reduce the magnetization to zero. The spin system is then allowed to recover for a variable length of time prior to applying a monitor pulse or pulse sequence, such as the CPMG sequence. The inversion recovery technique suggests that after the nuclei have aligned themselves along the static magnetic field, a 180° pulse is applied to reverse the direction of the spins. Over time, the spins decay toward their equilibrium direction according to T1, but no measurement is yet made as the 180° pulse does not induce a signal in the detector. Before the decay is complete, however, it is interrupted by a monitor pulse or pulse sequence, such as the CPMG sequence, which rotates the spins into the measurement plane (i.e., induces a signal in the detector). The information of interest is the amplitude of the signal immediately after the initial 90° “readout” pulse. This amplitude clearly depends on the recovery time between the initial 180° pulse and the 90° pulse. Following a determination of amplitude, the spin system is permitted to completely relax back to equilibrium, and the pulse sequence is then repeated.
An example of a downhole use of inversion recovery sequences is described in Kleinberg et. al, U.S. Pat. No. 5,023,551, entitled, “Nuclear Magnetic Resonance Pulse Sequences For Use With Borehole Logging Tools,” granted Jun. 11, 1991. However, the inversion recovery sequences described in the '551 patent do not use adiabatic pulses and therefore result in a narrow region of investigation. Also, under “inside-out” conditions in conjunction with motion, it may be easier to saturate a region than to invert it completely. Therefore, saturating a region may be preferred.
Referring back to FIG. 2, the T1 times typically are measured using polarization-based measurements instead of the decay-based measurements described above. In this manner, each polarization-based measurement may first include applying a saturation sequence to saturate the spins in a resonance region (such as the cylindrical resonance volume 20a as depicted in FIG. 2, for example). Subsequently, a polarization period elapses to allow polarization of the resonance volume 20a to the {right arrow over (B)}0 static magnetic field. Subsequently, a detection sequence, such as the CPMG sequence, is used to produce spin-echoes from the formation 10. The amplitudes of the first few spin-echoes are then analyzed to determine a polarization weighted integral Φ(twait) of the porosity distribution Φ(T1). Because only the first few echoes need to be observed to determine the amplitude of the signal, the T1 measurement may be performed in a shorter duration of time than the decay-based T2 measurement and thus, be less prone to motion of the NMR tool 6. The detection sequence may be successively repeated (after the appropriate saturation sequence) several times with varied wait times to obtain a porosity distribution Φ(T1).
As an example, a polarization-based measurement may be used to measure the T1 times for hydrogen nuclei in the resonance volume 20a located within the saturated volume 20b (see FIG. 2). In this manner, the NMR tool 6 may first saturate spins within the saturated volume 20b. However, the polarization period may be sufficiently long to permit the NMR tool 6 to significantly move within the borehole. In that case, tool 6 movement causes the resonance volume 20a to shift and causes the NMR tool 6 to receive spin-echoes from a shifted resonance volume 20a′ (see FIG. 3) that partially falls outside the original, saturated volume 20b. As a result, the shifted resonance volume 20a′ may comprise a region without saturated spins (an effect typically called “moving fresh spins in”) and a region of the original saturated volume 20b with saturated spins. Unfortunately, polarization-based NMR techniques may not be able to tolerate “fresh spins” being moved in during the polarization period, as the fresh spins may introduce measurement errors. For example, the measurements may erroneously indicate a higher bound fluid volume than is actually present in the formation.
One way to saturate a larger region is described in PCT Application Ser. No. PCT/US97/23975, entitled “Method For Formation Evaluation While Drilling,” filed on Dec. 29, 1997. This application discloses, at the start of a measurement, transmitting one or more radio frequency pulses covering a relatively wide range of frequencies and/or extra wide bandwidth or using one or more pulses which are frequency swept to saturate a cylindrical volume around an NMR tool. The application further describes the use of acceleration peak values to determine when to invalidate measurements due to movement of the tool beyond the extent of the saturated region, the application further describes fitting the tool with stand-offs to prevent movement of the tool beyond the saturated region.
Thus, there is a continuing need for minimizing error introduced by relative motion between an NMR measurement apparatus and a sample being investigated.